API Standards are provided as an engineering aid for specification and selection of "normal" and "emergency" pressure and vacuum Relief Valves for aboveground liquid petroleum storage tanks. Normal venting capacity is obtained without exceeding pressure or vacuum that would cause physical damage or permanent deformation to the tank. The following will help in sizing a Pressure/Vacuum Valve:

Normal Relief: The sum of vapor replacement resulting from emptying or filling and thermal in-breathing or outbreathing.

Emergency Relief: Thermal out-breathing from fire exposure.

All Tanks: Generally require the sizing of a normal pressure and vacuum Relief Valve to be sized and an independent emergency Relief Valve to be sized separately.

Flow Curves: These curves provide pressure and vacuum capacity which is required for sizing.